1. Field of the Invention
The present invention relates to an illumination optical apparatus equipped with an optical integrator, and more particularly, to an illumination optical apparatus that is suitable for use in the illumination system of a projection exposure apparatus used for lithographic processes in the manufacture of liquid crystal display panels and the like.
2. Discussion of the Related Art
In recent years, liquid crystal display panels have been widely used as display elements in personal computers and television sets, etc. Such liquid crystal display panels are manufactured by patterning transparent thin-film electrodes on a glass substrate into desired shapes by means of a photolithographic process. The photolithographic process employs a projection optical apparatus that exposes an original pattern (on a mask) onto a photo-resist layer on the glass substrate via a projection optical system.
Recently, there has been an increasing demand to expand the display area in the liquid crystal display panels. To cope with this trend, the exposure region of the projection exposure apparatus needs to be enlarged. A scan type exposure apparatus equipped with a plurality of projection optical systems has been proposed to expand the exposure region. In this system, an illuminating optical device having a plurality of illuminating optical systems is provided, and a mask is illuminated by light beams emitted from the respective illuminating optical systems so that images of the illuminated mask are projected onto the projection region of the glass substrate via the respective projection optical systems. Then, the entire area of the pattern region on the mask is transferred onto the glass substrate by scanning the mask and glass substrate in synchronization with each other with respect to the projection optical systems.
In order to simply and accurately control the amount of exposure in the plurality of illuminating optical systems in the above-mentioned scan type exposure apparatus, it is desirable that the illumination intensities in the respective illumination regions be uniform, and that the light intensities in the respective illumination regions be fixed during the scanning of the mask and glass substrate (or wafer). The applicant of the present invention previously disclosed in Japanese Laid-Open Patent Application No. 7-153683 (U.S. Pat. No. 5,581,075) an exposure apparatus in which a uniform illumination intensity is obtained at all the illumination regions by detecting the light beam produced from each of the illuminating optical systems of the projection optical systems, and by adjusting the illumination intensities of all the illuminating optical systems such that the adjusted illumination intensity matches the minimum intensity detected among the illumination regions.
In the exposure apparatus described above, a plurality of illuminating optical systems along with associated optical systems are arranged side by side in a row. Each of the illuminating optical systems include a light source having an elliptic mirror, a collimator lens system, a fly-eye lens, and a condenser lens system. Each of the associated optical system includes a mask, a projection optical system, and a substrate, in that order. An ND filter is installed between the light source and the collimator lens system in each optical system to adjust the intensity of the light beam of the corresponding illuminating optical system. Each ND filter includes a plurality of filters with different transmission coefficients and a filter driving device for engaging these filters. Each of the filters are inserted or retracted so as to generate a light beam with a desirable intensity for the respective illuminating optical system.
The ND filters of such an exposure apparatus generates a discrete variation in its light intensity, which is not suitable for fine tuning of the intensity. If the number of the ND filters is increased, this discontinuity can be reduced. However, in such a case, the overall size of the apparatus undesirably increases.
Meanwhile, in an effort to save energy consumption and reduce the overall size of the illuminating optical system, an exposure apparatus having a separate light source has been proposed. In this apparatus, light originating from the light source is divided into a plurality of light beams of equal intensity through a multi-branching random light guide and directed towards a plurality of illuminating optical systems. In this system, however, the light intensities in the respective illumination regions cannot be made sufficiently uniform due to variations in the transmissivity of the respective illuminating optical systems and projection optical systems, breakage of the optical fibers constituting the multi-branching random light guide, and other causes, such as variation in the manufacture of constituent parts. Accordingly, a light-intensity adjusting means as described above is indispensable in this case as well.